With remarkable advances in the performance of computers and other information equipment, the volume of information that is handled by users has been constantly increasing and the unit of measurement is now in terabytes rather than gigabytes. Under these circumstances, there exists an ever-growing demand for semiconductor devices such as information storage/reproduction equipment and memories that are capable of recording at even higher densities.
To achieve higher recording densities, technologies for even finer microfabrication are required. Conventional photolithography which uses the exposure process is capable of microfabrication over a large area in one step; however, since its resolution is not finer than the wavelength of light, conventional photolithography is inevitably unsuitable for creating fine structures smaller than the wavelength of light (say, 100 nm and less). Technologies currently available for processing finer structures than the wavelength of light include exposure using electron beams, exposure using X-rays, and exposure using ion beams. However, pattern formation with an electron beam lithographic apparatus differs from patterning by one-shot exposure using such light sources as i-line and an excimer laser in that the more patterns that need be written with electron beams, the longer the time that is required for writing (exposure). Therefore, as the recording density increases, the time it takes to form a fine-featured pattern is prolonged to cause a marked drop in throughput. With a view to forming patterns at a faster speed by the e-beam lithographic equipment, the development of a method for one-shot irradiation of geometric figures is underway in which combinations of variously shaped masks are subjected to one-shot exposure to electron beams; however, the e-beam lithographic apparatus that uses the method for one-shot irradiation of geometric figures is not only bulky but it also needs an additional mechanism for controlling the positions of masks to an even higher precision. This increases the cost of the lithographic apparatus, eventually leading to a higher cost for manufacturing media.
Printing-based approaches have been proposed as an alternative to the conventional exposure technologies for creating fine structures smaller than the wavelength of light. See, for example, U.S. Pat. No. 5,772,905A which describes an invention relating to the technology of nanoimprint lithography (NIL). The technology of nanoimprint lithography (NIL) is a technique in which a pattern of predetermined fine structures is formed on a mold by exposure to electron beams or using some other methods of creating finer structures than the wavelength of light and the mold is urged under pressure against a resist-coated transfer substrate so that the fine-structured pattern is transferred to the resist coating on the transfer substrate. As long as the mold is available, there is no particular need to employ an expensive exposure unit but an apparatus in the class of ordinary printing presses will suffice to produce replicas in large quantities; hence, in comparison with the conventional methods such as exposure to electron beams, there is achieved a marked improvement in throughput whereas the manufacturing cost is significantly reduced.
As described in U.S. Pat. No. 5,772,905A, when a thermoplastic resin (say, PMMA) is used as a resist material in the technology of nanoimprint lithography (NIL), transfer is performed with the thermoplastic resin being heated under pressure to a temperature near its glass transition temperature (Tg) or higher. This approach is called a heat transfer process. The heat transfer process has the advantage of permitting the use of general-purpose, thermoplastic resins. If a photosensitive resin is used as a resist in the NIL technology, a photocurable resin that hardens upon exposure to light such as UV radiation is chosen as the resin to which the original fine-featured pattern is transferred. This approach is called an optical transfer process.
In the nanoimprint processing technology using the optical transfer process, a special photocurable resin must be used but, on the other hand, compared to the heat transfer process, the optical transfer process has the advantage of reducing the dimensional errors in finished products due to the thermal expansion of transfer printing plates or printing media. Other advantages that are related to the apparatus include elimination of the need for equipping it with a heating mechanism and providing accessories such as for performing temperature elevation, temperature control, and cooling. There is a further advantage concerning the nanoimprint apparatus taken as a whole and that is elimination of the need for design considerations against thermal distortions, such as heat insulation.
An example of nanoimprint apparatuses based on the optical transfer process is described in JP2008-12844A. This apparatus is so designed that a stamper capable of transmitting UV light is urged against a photocurable resin coated transfer substrate and irradiated with UV light from above. A predetermined pattern of fine structures is formed in that surface of the stamper which is to be pressed against the transfer substrate.
FIGS. 6 (6(a) to 6(d)) in the accompanying drawings are schematic diagrams showing major steps in a fine-structure transfer method involving the nanoimprint technology based in the optical transfer process. In step (a), a transfer element 100 comprising a substrate 102 coated with a resist 104 on its topside is placed in a face-to-face relationship with a stamper 108 having a fine-featured pattern 106 formed on the side that is to be brought into contact with the resist 104. In step (b), the stamper 108 is pressed against the resist-coated surface of the transfer element 100. In step (c), ultraviolet (UV) light is applied to the stamper 108 from above, whereby the resist 104 is hardened. Then, in step (d), the stamper 108 is detached from the transfer element 100, leaving a patterned layer 110 on a surface of the substrate 102 of the transfer element 100. The patterned layer 110 is the obverse image of the fine-featured pattern 106.
In the nanoimprint technology, whether it is based on the heat transfer or optical transfer process, if the pressing of the stamper in the position shown in step (a) of FIG. 6 into intimate contact with the resist as shown in step (b) is performed in the atmosphere, air bubbles are trapped in the space between the recesses in the fine-featured pattern 106 on the stamper 108 and the resist 104 and even after the pressing action is finished, those air bubbles will stay, causing the external shape of the resist to be fixed as irregularly deformed. This makes it difficult or even impossible to ensure that the shape of the fine-featured pattern 106 is transferred correctly.
With a view to solving this problem of air bubbles, JP2008-12844A proposes a design as depicted in its accompanying FIG. 1, in which a plate on top of a stage on which a transfer element is to be placed has such a curved shape that it is highest in the central portion and becomes lower in a radial direction toward the outer periphery, with the result that when the stamper is pressed against the transfer element, the pressure applied to the transfer element is transmitted from the center outward, whereby air bubbles will be released to the outside of the transfer element. Briefly, pressure is applied with a certain gradient so that air as a compressive fluid is ejected from the outer edges of the transfer element. However, this method requires a great force to press the stamper.
FIG. 1 accompanying JP 2008-12858A depicts an imprint apparatus that contains a transfer element and a stamper within a vacuum chamber, the transfer element and the stamper being brought into contact with each other in a vacuum to thereby prevent any air bubbles from remaining between the stamper and the transfer element. However, this apparatus requires a vacuum chamber of complicated structure.
Further, FIG. 1 accompanying JP 2004-103817A depicts an imprint apparatus comprising a working compartment from which the gas in its interior can be evacuated by an exhaust device and into which a condensable gas of specified properties can be supplied from a condensable gas feeding device, further characterized in that after the space between a surface of the transfer element and a mold is filled by a condensable gas atmosphere, the mold is pressed against a resist layer on the surface of the transfer element to perform a transfer operation. A gas to be selected as the condensable gas of specified properties is such that it condenses when, during imprinting, the resist layer gets into the recesses in a surface of the mold to compress the gas in its interior, and it may be exemplified by trichlorofluoromethane. As a result of this condensation, the volume of the internal gas becomes negligibly small, preventing the generation of defects due to gas confinement in the recesses in a surface of the mold. However, since this apparatus requires a gas replacing means, the structure of the working compartment inevitably becomes complicated and bulky. As a further problem, the chemical effects the condensable gas used might have on the resist are yet to be studied and some unexpected inconvenience may potentially occur.